The discovery of galaxies

In the medieval world picture the stars were regarded as points of light attached to a sphere, whose surface was a long way outside the Solar System but whose volume was thought to be very much smaller than the space which we now know the stars to occupy. Attempts had indeed been made to determine the distance to the stellar sphere based on the possibility that stars might appear to be in different directions when observed from two points on the Earth's surface (fig. 1). Although this method worked for the Sun and Moon and other objects in the Solar System, it failed for the stars, indicating that they were very distant. After the Scientific Revolution in the 16th and 17th centuries, culminating in Newton's explanation of the motion of the planets in terms of a universal law of gravitation, it was realised that the stars were probably also suns or equivalently that the Sun was but one star amongst many and that the fixed stars should, in fact, be moving through space and should be influenced by the same law of gravitation. This led to a renewed interest in trying to determine not only their positions but also their motions.

Initially it was thought that there was just one system of stars filling the Universe and this view persisted until the second decade of this century.